1. Field of the Invention
The present invention relates to an electrophotographic image forming technique such as a printer, a copier machine and a facsimile machine, and more particularly, to an electrophotographic image forming technique which utilizes liquid development as a development method.
2. Description of the Related Art
Such an electrophotographic image forming apparatus has already been commercialized in which exposure means exposes a charged photosensitive member to thereby form an electrostatic latent image on the photosensitive member, developing means makes toner adhere to the photosensitive member, visualizes the electrostatic latent image and accordingly forms a toner image, and the toner image is then transferred onto a transfer medium. As a development method for the developing means, a liquid development method is known which uses a liquid developer which is obtained by dispersing toner in a carrier liquid. As advantages of the liquid development method, it is possible to obtain a high-resolution image owing to a small average particle diameter of toner which is as small as 0.1 through 2 μm, it is possible to obtain uniform images because of a high liquidity of the liquid developer, etc.
As known in the art, in this image forming apparatus, an image quality such as an image density can be controlled by changing image forming conditions which are defined by various factors, such as bias potentials applied upon respective portions of the apparatus. Further, the image density of a toner image may change depending on a difference between individual apparatuses, a change with time, a change in environment surrounding the apparatus such as a temperature and a humidity level, etc. Noting this, a density controlling technique has been proposed which controls an image density by adjusting an image forming condition which influences the image density among the factors mentioned above. In an apparatus described in Japanese Patent Application Laid-Open Gazette No. H8-292622 of 1996 for example, a test patch image is formed, light is irradiated upon the patch image, light from the patch image is received, the image density of the patch image is detected, and image forming conditions such as a surface potential of the photosensitive member and the toner density of the liquid developer are controlled based on the detected result.
By the way, while conventional image forming apparatuses have used a liquid developer having a relatively low toner density, liquid developers having relatively high toner densities have started to be used over the recent years for the following reasons. One of the reasons is that it is difficult to reduce the size of an apparatus since a large amount of a liquid developer is necessary to ensure a sufficient amount of toner as the liquid developer has a low toner density. Another reason is that the structure of the apparatus becomes complex in an effort to prevent a volatile solvent often used as a carrier liquid from leaking out of the apparatus. Further, when a liquid developer has a high toner density, the viscosity of the liquid developer increases. Hence, it is difficult to apply the density controlling technique described in Japanese Patent Application Laid-Open Gazette No. H8-292622 which is on a premise to use a liquid developer having a relatively low toner density directly to an image forming apparatus which uses a liquid developer having a relatively high toner density and a high viscosity.